Pellet mills with rollers that cooperate with a cylindrical die having a plurality of circular apertures are conventionally used to produce pellets for animal food or wood pellets that are used in furnaces or wood burning stoves, which for example are used in domestic or district heating systems.
A problem with the commonly used pellet mills is that the pelletizable material is fed directly into the die cylinder, and therefore due to the influence of gravity has a tendency to accumulate in the “bottom” of the die chamber. This causes large unbalances in the pellet mill, because not all rollers are fed the same quantity of material, which material furthermore may be unevenly distributed along the width of the roller. These unbalances lead to an increased wear of the structural elements of the pellet mill, especially its bearings. Ultimately, this will lead to increased maintenance work on the pellet mill, which is unproductive while being maintained, and greatly reduces the lifetime of it.
To overcome these difficulties, a number of solutions have been suggested. For example in U.S. Pat. No. 4,162,881 is disclosed a pellet mill with two side-by-side rollers which are disposed in respective first and second hemispheres (divided along a vertical center line) of rotation of a vertical rotatable cylindrical pellet die having a plurality of apertures. Pelletizable material is fed in independent first and second conduits for each roller. The first conduit deposits the pelletizable material in the first hemisphere above the first roller, while the second conduit supplies the pelletizable material also in the first hemisphere of rotation below the first roller. Thus, such material is initially deposited on the die member from the conduit and is carried during rotation by centrifugal force to the second roller. The pelletizable material is not fed directly to a wedge-shaped space between a roller and the inner cylindrical surface of the die. This implies that not all the material fed by the second conduit is carried to the second roller. The material has therefore a tendency to build up in the bottom of the die chamber, which again will lead to the unbalances discussed above. Another disadvantage of this pellet mill is that it does not permit the use of a traditional shear pin safety mechanism, because the conduits would prevent the free rotation of the rollers, if the shear pin is broken.
In U.S. Pat. No. 3,932,091 is disclosed a pellet mill for making food pellets from a flour product. The mill comprises a number of feed tubes, one for each extrusion roller, which tubes each extend axially through the interior of the annular die between the rollers and near the inner surface of the annular die. The flour product is fed to these tubes by screw conveyors. Each feed tube has a side opening having the same axial length as the associated roller and having a circumferential width which gradually increases inwardly. It is alleged that this leads to an even distribution of the flour product over the length of the roller and the effective width of the annular die.
In EP 0371 519 is disclosed a pellet mill with two rollers mounted on roller shafts having an eccentric stud. The rollers are also provided with cylinder and piston means for adjusting the distance between the rollers and the inner cylindrical surface of the die. These cylinder and piston means are provided with internal position sensors for measuring said distance.
In FR 2 548 957 is disclosed a pellet mill employing a shear point safety pin, which is configured to shear in the event of an accidental blocking of the system.
Furthermore, the documents U.S. Pat. Nos. 3,807,926, 4,711,622, 3,045,280 and GB 857,133 disclose various feeding mechanisms for pellet mills.